1. Field of Invention
The present invention relates to power converters, and more particularly to, to over-current protection for a power converter.
2. Description of Related Art
Power converters are essential for many modern electronic devices. Among other capabilities, power converters can adjust voltage level downward (buck converter) or adjust voltage level upward (boost converter). Power converters may also convert from alternating current (AC) power to direct current (DC) power, or vice versa. Power converters are typically implemented using one or more switching devices, such as transistors, which are turned on and off to deliver power to the output of the converter. Control circuitry is provided to regulate the turning on and off of the switching devices, and thus, these converters are known as “switching regulators” or “switching converters.” The power converters may also include one or more capacitors or inductors for alternately storing and outputting energy.
Short circuit protection in DC/DC switching regulators is necessary for protecting against catastrophic failures due to current ratcheting. Current ratcheting may occur in switching regulators using current mode control because the control loop takes a finite time to react. For example, in a DC/DC step-down switching regulator, when the output is shorted, almost the full voltage of the supply will appear across the inductor for a short period of time. The inductor current climbs or increases at a very fast rate. The internal current sense loop requires a certain amount of time to respond. When the current sense loop finally detects that the current is too high, there will be almost zero voltage across the inductor, which makes the current in the inductor decay very slowly. This process, which continues at the rate of the switching frequency, causes a “ratcheting” of the inductor current to very high levels—sometimes with catastrophic outcomes, such as a complete breakdown of the switching regulator. The peak amount of current in a current ratcheting situation depends on the switching frequency and loop speed of the entire circuit.
Various techniques according to previously developed designs provide short circuit protection by either employing frequency foldback (which reduces the average current in the switching regulator) or by providing a secondary current limit in the switching regulator. With frequency foldback when the output voltage of a switching regulator is fractionally lower than the intended voltage, the frequency of the regulator is slowed down to reduce the total amount of current. The lower frequency allows for more of the current in the inductor to discharge so that the average current is lower. Although the average current may be lower, the peak currents in the regulator remain the same. As such, larger input and output capacitors are still required due to voltage ripple concerns. Furthermore, the use of lower frequencies creates lower harmonics, which requires additional circuitry in the switching regulator to address. With techniques which provide a secondary current limit, the switching regulator is shut down when the current exceeds a certain threshold which is higher than the primary current limit. The switching regulator may restart after a certain amount of time has passed. Such techniques can be problematic because the secondary current limit may be higher than the maximum specified current rating for some applications. Also, both techniques (i.e., frequency foldback and secondary current limit) may increase component values.